X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FCodeGen%2FBranchFolding.cpp;h=dc67593c339aa93709ce1e5115d5975e29388433;hb=afe6c2b001a924cd74bd0aacfed5984d9af004b0;hp=806f54da7856db3ac99785b6c3da60eaddf287d1;hpb=8f16eb98ed21a642d7f75c76f4b1acf0f199ee6d;p=oota-llvm.git diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp index 806f54da785..dc67593c339 100644 --- a/lib/CodeGen/BranchFolding.cpp +++ b/lib/CodeGen/BranchFolding.cpp @@ -2,8 +2,8 @@ // // The LLVM Compiler Infrastructure // -// This file was developed by the LLVM research group and is distributed under -// the University of Illinois Open Source License. See LICENSE.TXT for details. +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // @@ -16,257 +16,1084 @@ // //===----------------------------------------------------------------------===// +#define DEBUG_TYPE "branchfolding" #include "llvm/CodeGen/Passes.h" +#include "llvm/CodeGen/MachineModuleInfo.h" #include "llvm/CodeGen/MachineFunctionPass.h" +#include "llvm/CodeGen/MachineJumpTableInfo.h" +#include "llvm/CodeGen/RegisterScavenging.h" #include "llvm/Target/TargetInstrInfo.h" #include "llvm/Target/TargetMachine.h" +#include "llvm/Target/TargetRegisterInfo.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/ADT/Statistic.h" #include "llvm/ADT/STLExtras.h" +#include using namespace llvm; +STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); +STATISTIC(NumBranchOpts, "Number of branches optimized"); +STATISTIC(NumTailMerge , "Number of block tails merged"); +static cl::opt FlagEnableTailMerge("enable-tail-merge", + cl::init(cl::BOU_UNSET), cl::Hidden); namespace { + // Throttle for huge numbers of predecessors (compile speed problems) + cl::opt + TailMergeThreshold("tail-merge-threshold", + cl::desc("Max number of predecessors to consider tail merging"), + cl::init(100), cl::Hidden); + struct BranchFolder : public MachineFunctionPass { + static char ID; + explicit BranchFolder(bool defaultEnableTailMerge) : + MachineFunctionPass((intptr_t)&ID) { + switch (FlagEnableTailMerge) { + case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; + case cl::BOU_TRUE: EnableTailMerge = true; break; + case cl::BOU_FALSE: EnableTailMerge = false; break; + } + } + virtual bool runOnMachineFunction(MachineFunction &MF); virtual const char *getPassName() const { return "Control Flow Optimizer"; } const TargetInstrInfo *TII; + MachineModuleInfo *MMI; bool MadeChange; private: - void OptimizeBlock(MachineFunction::iterator MBB); + // Tail Merging. + bool EnableTailMerge; + bool TailMergeBlocks(MachineFunction &MF); + bool TryMergeBlocks(MachineBasicBlock* SuccBB, + MachineBasicBlock* PredBB); + void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, + MachineBasicBlock *NewDest); + MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, + MachineBasicBlock::iterator BBI1); + + std::vector > MergePotentials; + const TargetRegisterInfo *RegInfo; + RegScavenger *RS; + // Branch optzn. + bool OptimizeBranches(MachineFunction &MF); + void OptimizeBlock(MachineBasicBlock *MBB); + void RemoveDeadBlock(MachineBasicBlock *MBB); + + bool CanFallThrough(MachineBasicBlock *CurBB); + bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, + MachineBasicBlock *TBB, MachineBasicBlock *FBB, + const std::vector &Cond); }; + char BranchFolder::ID = 0; } -FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); } +FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { + return new BranchFolder(DefaultEnableTailMerge); } + +/// RemoveDeadBlock - Remove the specified dead machine basic block from the +/// function, updating the CFG. +void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { + assert(MBB->pred_empty() && "MBB must be dead!"); + DOUT << "\nRemoving MBB: " << *MBB; + + MachineFunction *MF = MBB->getParent(); + // drop all successors. + while (!MBB->succ_empty()) + MBB->removeSuccessor(MBB->succ_end()-1); + + // If there is DWARF info to active, check to see if there are any LABEL + // records in the basic block. If so, unregister them from MachineModuleInfo. + if (MMI && !MBB->empty()) { + for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); + I != E; ++I) { + if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) { + // The label ID # is always operand #0, an immediate. + MMI->InvalidateLabel(I->getOperand(0).getImm()); + } + } + } + + // Remove the block. + MF->getBasicBlockList().erase(MBB); +} bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { TII = MF.getTarget().getInstrInfo(); if (!TII) return false; + // Fix CFG. The later algorithms expect it to be right. + bool EverMadeChange = false; + for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { + MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; + std::vector Cond; + if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond)) + EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); + } + + RegInfo = MF.getTarget().getRegisterInfo(); + RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; + + MMI = getAnalysisToUpdate(); + + bool MadeChangeThisIteration = true; + while (MadeChangeThisIteration) { + MadeChangeThisIteration = false; + MadeChangeThisIteration |= TailMergeBlocks(MF); + MadeChangeThisIteration |= OptimizeBranches(MF); + EverMadeChange |= MadeChangeThisIteration; + } + + // See if any jump tables have become mergable or dead as the code generator + // did its thing. + MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); + const std::vector &JTs = JTI->getJumpTables(); + if (!JTs.empty()) { + // Figure out how these jump tables should be merged. + std::vector JTMapping; + JTMapping.reserve(JTs.size()); + + // We always keep the 0th jump table. + JTMapping.push_back(0); + + // Scan the jump tables, seeing if there are any duplicates. Note that this + // is N^2, which should be fixed someday. + for (unsigned i = 1, e = JTs.size(); i != e; ++i) + JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); + + // If a jump table was merge with another one, walk the function rewriting + // references to jump tables to reference the new JT ID's. Keep track of + // whether we see a jump table idx, if not, we can delete the JT. + std::vector JTIsLive; + JTIsLive.resize(JTs.size()); + for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); + BB != E; ++BB) { + for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); + I != E; ++I) + for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { + MachineOperand &Op = I->getOperand(op); + if (!Op.isJumpTableIndex()) continue; + unsigned NewIdx = JTMapping[Op.getIndex()]; + Op.setIndex(NewIdx); + + // Remember that this JT is live. + JTIsLive[NewIdx] = true; + } + } + + // Finally, remove dead jump tables. This happens either because the + // indirect jump was unreachable (and thus deleted) or because the jump + // table was merged with some other one. + for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) + if (!JTIsLive[i]) { + JTI->RemoveJumpTable(i); + EverMadeChange = true; + } + } - // DISABLED FOR NOW. - return false; + delete RS; + return EverMadeChange; +} + +//===----------------------------------------------------------------------===// +// Tail Merging of Blocks +//===----------------------------------------------------------------------===// + +/// HashMachineInstr - Compute a hash value for MI and its operands. +static unsigned HashMachineInstr(const MachineInstr *MI) { + unsigned Hash = MI->getOpcode(); + for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { + const MachineOperand &Op = MI->getOperand(i); + + // Merge in bits from the operand if easy. + unsigned OperandHash = 0; + switch (Op.getType()) { + case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; + case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; + case MachineOperand::MO_MachineBasicBlock: + OperandHash = Op.getMBB()->getNumber(); + break; + case MachineOperand::MO_FrameIndex: + case MachineOperand::MO_ConstantPoolIndex: + case MachineOperand::MO_JumpTableIndex: + OperandHash = Op.getIndex(); + break; + case MachineOperand::MO_GlobalAddress: + case MachineOperand::MO_ExternalSymbol: + // Global address / external symbol are too hard, don't bother, but do + // pull in the offset. + OperandHash = Op.getOffset(); + break; + default: break; + } + + Hash += ((OperandHash << 3) | Op.getType()) << (i&31); + } + return Hash; +} + +/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks +/// with no successors, we hash two instructions, because cross-jumping +/// only saves code when at least two instructions are removed (since a +/// branch must be inserted). For blocks with a successor, one of the +/// two blocks to be tail-merged will end with a branch already, so +/// it gains to cross-jump even for one instruction. + +static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, + unsigned minCommonTailLength) { + MachineBasicBlock::const_iterator I = MBB->end(); + if (I == MBB->begin()) + return 0; // Empty MBB. - //MF.dump(); + --I; + unsigned Hash = HashMachineInstr(I); + + if (I == MBB->begin() || minCommonTailLength == 1) + return Hash; // Single instr MBB. - bool EverMadeChange = false; - MadeChange = true; - while (MadeChange) { - MadeChange = false; - for (MachineFunction::iterator MBB = ++MF.begin(), E = MF.end(); MBB != E; - ++MBB) - OptimizeBlock(MBB); + --I; + // Hash in the second-to-last instruction. + Hash ^= HashMachineInstr(I) << 2; + return Hash; +} + +/// ComputeCommonTailLength - Given two machine basic blocks, compute the number +/// of instructions they actually have in common together at their end. Return +/// iterators for the first shared instruction in each block. +static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, + MachineBasicBlock *MBB2, + MachineBasicBlock::iterator &I1, + MachineBasicBlock::iterator &I2) { + I1 = MBB1->end(); + I2 = MBB2->end(); + + unsigned TailLen = 0; + while (I1 != MBB1->begin() && I2 != MBB2->begin()) { + --I1; --I2; + if (!I1->isIdenticalTo(I2) || + // FIXME: This check is dubious. It's used to get around a problem where + // people incorrectly expect inline asm directives to remain in the same + // relative order. This is untenable because normal compiler + // optimizations (like this one) may reorder and/or merge these + // directives. + I1->getOpcode() == TargetInstrInfo::INLINEASM) { + ++I1; ++I2; + break; + } + ++TailLen; + } + return TailLen; +} + +/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything +/// after it, replacing it with an unconditional branch to NewDest. This +/// returns true if OldInst's block is modified, false if NewDest is modified. +void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, + MachineBasicBlock *NewDest) { + MachineBasicBlock *OldBB = OldInst->getParent(); + + // Remove all the old successors of OldBB from the CFG. + while (!OldBB->succ_empty()) + OldBB->removeSuccessor(OldBB->succ_begin()); + + // Remove all the dead instructions from the end of OldBB. + OldBB->erase(OldInst, OldBB->end()); + + // If OldBB isn't immediately before OldBB, insert a branch to it. + if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) + TII->InsertBranch(*OldBB, NewDest, 0, std::vector()); + OldBB->addSuccessor(NewDest); + ++NumTailMerge; +} + +/// SplitMBBAt - Given a machine basic block and an iterator into it, split the +/// MBB so that the part before the iterator falls into the part starting at the +/// iterator. This returns the new MBB. +MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, + MachineBasicBlock::iterator BBI1) { + // Create the fall-through block. + MachineFunction::iterator MBBI = &CurMBB; + MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); + CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); + + // Move all the successors of this block to the specified block. + while (!CurMBB.succ_empty()) { + MachineBasicBlock *S = *(CurMBB.succ_end()-1); + NewMBB->addSuccessor(S); + CurMBB.removeSuccessor(S); + } + + // Add an edge from CurMBB to NewMBB for the fall-through. + CurMBB.addSuccessor(NewMBB); + + // Splice the code over. + NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); + + // For targets that use the register scavenger, we must maintain LiveIns. + if (RS) { + RS->enterBasicBlock(&CurMBB); + if (!CurMBB.empty()) + RS->forward(prior(CurMBB.end())); + BitVector RegsLiveAtExit(RegInfo->getNumRegs()); + RS->getRegsUsed(RegsLiveAtExit, false); + for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) + if (RegsLiveAtExit[i]) + NewMBB->addLiveIn(i); + } + + return NewMBB; +} + +/// EstimateRuntime - Make a rough estimate for how long it will take to run +/// the specified code. +static unsigned EstimateRuntime(MachineBasicBlock::iterator I, + MachineBasicBlock::iterator E) { + unsigned Time = 0; + for (; I != E; ++I) { + const TargetInstrDesc &TID = I->getDesc(); + if (TID.isCall()) + Time += 10; + else if (TID.isSimpleLoad() || TID.mayStore()) + Time += 2; + else + ++Time; + } + return Time; +} + +/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at +/// MBB2I and then insert an unconditional branch in the other block. Determine +/// which is the best to split +static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, + MachineBasicBlock::iterator MBB1I, + MachineBasicBlock *MBB2, + MachineBasicBlock::iterator MBB2I, + MachineBasicBlock *PredBB) { + // If one block is the entry block, split the other one; we can't generate + // a branch to the entry block, as its label is not emitted. + MachineBasicBlock *Entry = MBB1->getParent()->begin(); + if (MBB1 == Entry) + return false; + if (MBB2 == Entry) + return true; + + // If one block falls through into the common successor, choose that + // one to split; it is one instruction less to do that. + if (PredBB) { + if (MBB1 == PredBB) + return true; + else if (MBB2 == PredBB) + return false; + } + // TODO: if we had some notion of which block was hotter, we could split + // the hot block, so it is the fall-through. Since we don't have profile info + // make a decision based on which will hurt most to split. + unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I); + unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I); + + // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the + // MBB1 block so it falls through. This will penalize the MBB2 path, but will + // have a lower overall impact on the program execution. + return MBB1Time < MBB2Time; +} + +// CurMBB needs to add an unconditional branch to SuccMBB (we removed these +// branches temporarily for tail merging). In the case where CurMBB ends +// with a conditional branch to the next block, optimize by reversing the +// test and conditionally branching to SuccMBB instead. + +static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, + const TargetInstrInfo *TII) { + MachineFunction *MF = CurMBB->getParent(); + MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); + MachineBasicBlock *TBB = 0, *FBB = 0; + std::vector Cond; + if (I != MF->end() && + !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) { + MachineBasicBlock *NextBB = I; + if (TBB == NextBB && Cond.size() && !FBB) { + if (!TII->ReverseBranchCondition(Cond)) { + TII->RemoveBranch(*CurMBB); + TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); + return; + } + } + } + TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector()); +} + +static bool MergeCompare(const std::pair &p, + const std::pair &q) { + if (p.first < q.first) + return true; + else if (p.first > q.first) + return false; + else if (p.second->getNumber() < q.second->getNumber()) + return true; + else if (p.second->getNumber() > q.second->getNumber()) + return false; + else { + // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing + // an object with itself. +#ifndef _GLIBCXX_DEBUG + assert(0 && "Predecessor appears twice"); +#endif + return(false); + } +} + +// See if any of the blocks in MergePotentials (which all have a common single +// successor, or all have no successor) can be tail-merged. If there is a +// successor, any blocks in MergePotentials that are not tail-merged and +// are not immediately before Succ must have an unconditional branch to +// Succ added (but the predecessor/successor lists need no adjustment). +// The lone predecessor of Succ that falls through into Succ, +// if any, is given in PredBB. + +bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, + MachineBasicBlock* PredBB) { + unsigned minCommonTailLength = (SuccBB ? 1 : 2); + MadeChange = false; + + // Sort by hash value so that blocks with identical end sequences sort + // together. + std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare); + + // Walk through equivalence sets looking for actual exact matches. + while (MergePotentials.size() > 1) { + unsigned CurHash = (MergePotentials.end()-1)->first; + unsigned PrevHash = (MergePotentials.end()-2)->first; + MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; + + // If there is nothing that matches the hash of the current basic block, + // give up. + if (CurHash != PrevHash) { + if (SuccBB && CurMBB != PredBB) + FixTail(CurMBB, SuccBB, TII); + MergePotentials.pop_back(); + continue; + } - // If branches were folded away somehow, do a quick scan and delete any dead - // blocks. - if (MadeChange) { - for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { - MachineBasicBlock *MBB = I++; - // Is it dead? - if (MBB->pred_empty()) { - // drop all successors. - while (!MBB->succ_empty()) - MBB->removeSuccessor(MBB->succ_end()-1); - MF.getBasicBlockList().erase(MBB); + // Look through all the pairs of blocks that have the same hash as this + // one, and find the pair that has the largest number of instructions in + // common. + // Since instructions may get combined later (e.g. single stores into + // store multiple) this measure is not particularly accurate. + MachineBasicBlock::iterator BBI1, BBI2; + + unsigned FoundI = ~0U, FoundJ = ~0U; + unsigned maxCommonTailLength = 0U; + for (int i = MergePotentials.size()-1; + i != -1 && MergePotentials[i].first == CurHash; --i) { + for (int j = i-1; + j != -1 && MergePotentials[j].first == CurHash; --j) { + MachineBasicBlock::iterator TrialBBI1, TrialBBI2; + unsigned CommonTailLen = ComputeCommonTailLength( + MergePotentials[i].second, + MergePotentials[j].second, + TrialBBI1, TrialBBI2); + if (CommonTailLen >= minCommonTailLength && + CommonTailLen > maxCommonTailLength) { + FoundI = i; + FoundJ = j; + maxCommonTailLength = CommonTailLen; + BBI1 = TrialBBI1; + BBI2 = TrialBBI2; } } } - EverMadeChange |= MadeChange; - } + // If we didn't find any pair that has at least minCommonTailLength + // instructions in common, bail out. All entries with this + // hash code can go away now. + if (FoundI == ~0U) { + for (int i = MergePotentials.size()-1; + i != -1 && MergePotentials[i].first == CurHash; --i) { + // Put the unconditional branch back, if we need one. + CurMBB = MergePotentials[i].second; + if (SuccBB && CurMBB != PredBB) + FixTail(CurMBB, SuccBB, TII); + MergePotentials.pop_back(); + } + continue; + } - return EverMadeChange; + // Otherwise, move the block(s) to the right position(s). So that + // BBI1/2 will be valid, the last must be I and the next-to-last J. + if (FoundI != MergePotentials.size()-1) + std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1)); + if (FoundJ != MergePotentials.size()-2) + std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2)); + + CurMBB = (MergePotentials.end()-1)->second; + MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; + + // If neither block is the entire common tail, split the tail of one block + // to make it redundant with the other tail. Also, we cannot jump to the + // entry block, so if one block is the entry block, split the other one. + MachineBasicBlock *Entry = CurMBB->getParent()->begin(); + if (CurMBB->begin() == BBI1 && CurMBB != Entry) + ; // CurMBB is common tail + else if (MBB2->begin() == BBI2 && MBB2 != Entry) + ; // MBB2 is common tail + else { + if (0) { // Enable this to disable partial tail merges. + MergePotentials.pop_back(); + continue; + } + + // Decide whether we want to split CurMBB or MBB2. + if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, PredBB)) { + CurMBB = SplitMBBAt(*CurMBB, BBI1); + BBI1 = CurMBB->begin(); + MergePotentials.back().second = CurMBB; + } else { + MBB2 = SplitMBBAt(*MBB2, BBI2); + BBI2 = MBB2->begin(); + (MergePotentials.end()-2)->second = MBB2; + } + } + + if (MBB2->begin() == BBI2 && MBB2 != Entry) { + // Hack the end off CurMBB, making it jump to MBBI@ instead. + ReplaceTailWithBranchTo(BBI1, MBB2); + // This modifies CurMBB, so remove it from the worklist. + MergePotentials.pop_back(); + } else { + assert(CurMBB->begin() == BBI1 && CurMBB != Entry && + "Didn't split block correctly?"); + // Hack the end off MBB2, making it jump to CurMBB instead. + ReplaceTailWithBranchTo(BBI2, CurMBB); + // This modifies MBB2, so remove it from the worklist. + MergePotentials.erase(MergePotentials.end()-2); + } + MadeChange = true; + } + return MadeChange; } -/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to -/// 'Old', change the code and CFG so that it branches to 'New' instead. -static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB, - MachineBasicBlock *Old, - MachineBasicBlock *New, - const TargetInstrInfo *TII) { - assert(Old != New && "Cannot replace self with self!"); - - MachineBasicBlock::iterator I = BB->end(); - while (I != BB->begin()) { - --I; - if (!TII->isTerminatorInstr(I->getOpcode())) break; - - // Scan the operands of this machine instruction, replacing any uses of Old - // with New. - for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) - if (I->getOperand(i).isMachineBasicBlock() && - I->getOperand(i).getMachineBasicBlock() == Old) - I->getOperand(i).setMachineBasicBlock(New); +bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { + + if (!EnableTailMerge) return false; + + MadeChange = false; + + // First find blocks with no successors. + MergePotentials.clear(); + for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { + if (I->succ_empty()) + MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); } + // See if we can do any tail merging on those. + if (MergePotentials.size() < TailMergeThreshold) + MadeChange |= TryMergeBlocks(NULL, NULL); + + // Look at blocks (IBB) with multiple predecessors (PBB). + // We change each predecessor to a canonical form, by + // (1) temporarily removing any unconditional branch from the predecessor + // to IBB, and + // (2) alter conditional branches so they branch to the other block + // not IBB; this may require adding back an unconditional branch to IBB + // later, where there wasn't one coming in. E.g. + // Bcc IBB + // fallthrough to QBB + // here becomes + // Bncc QBB + // with a conceptual B to IBB after that, which never actually exists. + // With those changes, we see whether the predecessors' tails match, + // and merge them if so. We change things out of canonical form and + // back to the way they were later in the process. (OptimizeBranches + // would undo some of this, but we can't use it, because we'd get into + // a compile-time infinite loop repeatedly doing and undoing the same + // transformations.) - // Update the successor information. - std::vector Succs(BB->succ_begin(), BB->succ_end()); - for (int i = Succs.size()-1; i >= 0; --i) - if (Succs[i] == Old) { - BB->removeSuccessor(Old); - BB->addSuccessor(New); + for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { + if (!I->succ_empty() && I->pred_size() >= 2 && + I->pred_size() < TailMergeThreshold) { + MachineBasicBlock *IBB = I; + MachineBasicBlock *PredBB = prior(I); + MergePotentials.clear(); + for (MachineBasicBlock::pred_iterator P = I->pred_begin(), + E2 = I->pred_end(); + P != E2; ++P) { + MachineBasicBlock* PBB = *P; + // Skip blocks that loop to themselves, can't tail merge these. + if (PBB==IBB) + continue; + MachineBasicBlock *TBB = 0, *FBB = 0; + std::vector Cond; + if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) { + // Failing case: IBB is the target of a cbr, and + // we cannot reverse the branch. + std::vector NewCond(Cond); + if (Cond.size() && TBB==IBB) { + if (TII->ReverseBranchCondition(NewCond)) + continue; + // This is the QBB case described above + if (!FBB) + FBB = next(MachineFunction::iterator(PBB)); + } + // Failing case: the only way IBB can be reached from PBB is via + // exception handling. Happens for landing pads. Would be nice + // to have a bit in the edge so we didn't have to do all this. + if (IBB->isLandingPad()) { + MachineFunction::iterator IP = PBB; IP++; + MachineBasicBlock* PredNextBB = NULL; + if (IP!=MF.end()) + PredNextBB = IP; + if (TBB==NULL) { + if (IBB!=PredNextBB) // fallthrough + continue; + } else if (FBB) { + if (TBB!=IBB && FBB!=IBB) // cbr then ubr + continue; + } else if (Cond.empty()) { + if (TBB!=IBB) // ubr + continue; + } else { + if (TBB!=IBB && IBB!=PredNextBB) // cbr + continue; + } + } + // Remove the unconditional branch at the end, if any. + if (TBB && (Cond.size()==0 || FBB)) { + TII->RemoveBranch(*PBB); + if (Cond.size()) + // reinsert conditional branch only, for now + TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); + } + MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); + } + } + if (MergePotentials.size() >= 2) + MadeChange |= TryMergeBlocks(I, PredBB); + // Reinsert an unconditional branch if needed. + // The 1 below can be either an original single predecessor, or a result + // of removing blocks in TryMergeBlocks. + PredBB = prior(I); // this may have been changed in TryMergeBlocks + if (MergePotentials.size()==1 && + (MergePotentials.begin())->second != PredBB) + FixTail((MergePotentials.begin())->second, I, TII); } + } + return MadeChange; +} + +//===----------------------------------------------------------------------===// +// Branch Optimization +//===----------------------------------------------------------------------===// + +bool BranchFolder::OptimizeBranches(MachineFunction &MF) { + MadeChange = false; + + // Make sure blocks are numbered in order + MF.RenumberBlocks(); + + for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { + MachineBasicBlock *MBB = I++; + OptimizeBlock(MBB); + + // If it is dead, remove it. + if (MBB->pred_empty()) { + RemoveDeadBlock(MBB); + MadeChange = true; + ++NumDeadBlocks; + } + } + return MadeChange; +} + + +/// CanFallThrough - Return true if the specified block (with the specified +/// branch condition) can implicitly transfer control to the block after it by +/// falling off the end of it. This should return false if it can reach the +/// block after it, but it uses an explicit branch to do so (e.g. a table jump). +/// +/// True is a conservative answer. +/// +bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, + bool BranchUnAnalyzable, + MachineBasicBlock *TBB, MachineBasicBlock *FBB, + const std::vector &Cond) { + MachineFunction::iterator Fallthrough = CurBB; + ++Fallthrough; + // If FallthroughBlock is off the end of the function, it can't fall through. + if (Fallthrough == CurBB->getParent()->end()) + return false; + + // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. + if (!CurBB->isSuccessor(Fallthrough)) + return false; + + // If we couldn't analyze the branch, assume it could fall through. + if (BranchUnAnalyzable) return true; + + // If there is no branch, control always falls through. + if (TBB == 0) return true; + + // If there is some explicit branch to the fallthrough block, it can obviously + // reach, even though the branch should get folded to fall through implicitly. + if (MachineFunction::iterator(TBB) == Fallthrough || + MachineFunction::iterator(FBB) == Fallthrough) + return true; + + // If it's an unconditional branch to some block not the fall through, it + // doesn't fall through. + if (Cond.empty()) return false; + + // Otherwise, if it is conditional and has no explicit false block, it falls + // through. + return FBB == 0; +} + +/// CanFallThrough - Return true if the specified can implicitly transfer +/// control to the block after it by falling off the end of it. This should +/// return false if it can reach the block after it, but it uses an explicit +/// branch to do so (e.g. a table jump). +/// +/// True is a conservative answer. +/// +bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { + MachineBasicBlock *TBB = 0, *FBB = 0; + std::vector Cond; + bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); + return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); +} + +/// IsBetterFallthrough - Return true if it would be clearly better to +/// fall-through to MBB1 than to fall through into MBB2. This has to return +/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will +/// result in infinite loops. +static bool IsBetterFallthrough(MachineBasicBlock *MBB1, + MachineBasicBlock *MBB2) { + // Right now, we use a simple heuristic. If MBB2 ends with a call, and + // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to + // optimize branches that branch to either a return block or an assert block + // into a fallthrough to the return. + if (MBB1->empty() || MBB2->empty()) return false; + + // If there is a clear successor ordering we make sure that one block + // will fall through to the next + if (MBB1->isSuccessor(MBB2)) return true; + if (MBB2->isSuccessor(MBB1)) return false; + + MachineInstr *MBB1I = --MBB1->end(); + MachineInstr *MBB2I = --MBB2->end(); + return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); } /// OptimizeBlock - Analyze and optimize control flow related to the specified /// block. This is never called on the entry block. -void BranchFolder::OptimizeBlock(MachineFunction::iterator MBB) { +void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { + MachineFunction::iterator FallThrough = MBB; + ++FallThrough; + // If this block is empty, make everyone use its fall-through, not the block - // explicitly. - if (MBB->empty()) { - if (MBB->pred_empty()) return; // dead block? Leave for cleanup later. - - MachineFunction::iterator FallThrough = next(MBB); + // explicitly. Landing pads should not do this since the landing-pad table + // points to this block. + if (MBB->empty() && !MBB->isLandingPad()) { + // Dead block? Leave for cleanup later. + if (MBB->pred_empty()) return; - if (FallThrough != MBB->getParent()->end()) { + if (FallThrough == MBB->getParent()->end()) { + // TODO: Simplify preds to not branch here if possible! + } else { + // Rewrite all predecessors of the old block to go to the fallthrough + // instead. while (!MBB->pred_empty()) { MachineBasicBlock *Pred = *(MBB->pred_end()-1); - ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII); + Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); } + + // If MBB was the target of a jump table, update jump tables to go to the + // fallthrough instead. + MBB->getParent()->getJumpTableInfo()-> + ReplaceMBBInJumpTables(MBB, FallThrough); MadeChange = true; } - // TODO: CHANGE STUFF TO NOT BRANCH HERE! return; } // Check to see if we can simplify the terminator of the block before this // one. -#if 0 + MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); + MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; std::vector PriorCond; - if (!TII->AnalyzeBranch(*prior(MBB), PriorTBB, PriorFBB, PriorCond)) { + bool PriorUnAnalyzable = + TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); + if (!PriorUnAnalyzable) { + // If the CFG for the prior block has extra edges, remove them. + MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, + !PriorCond.empty()); + // If the previous branch is conditional and both conditions go to the same - // destination, remove the branch, replacing it with an unconditional one. + // destination, remove the branch, replacing it with an unconditional one or + // a fall-through. if (PriorTBB && PriorTBB == PriorFBB) { - TII->RemoveBranch(*prior(MBB)); + TII->RemoveBranch(PrevBB); PriorCond.clear(); - if (PriorTBB != &*MBB) - TII->InsertBranch(*prior(MBB), PriorTBB, 0, PriorCond); + if (PriorTBB != MBB) + TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); MadeChange = true; + ++NumBranchOpts; return OptimizeBlock(MBB); } // If the previous branch *only* branches to *this* block (conditional or // not) remove the branch. - if (PriorTBB == &*MBB && PriorFBB == 0) { - TII->RemoveBranch(*prior(MBB)); + if (PriorTBB == MBB && PriorFBB == 0) { + TII->RemoveBranch(PrevBB); MadeChange = true; + ++NumBranchOpts; return OptimizeBlock(MBB); } - } -#endif - - -#if 0 - - if (MBB->pred_size() == 1) { - // If this block has a single predecessor, and if that block has a single - // successor, merge this block into that block. - MachineBasicBlock *Pred = *MBB->pred_begin(); - if (Pred->succ_size() == 1) { - // Delete all of the terminators from end of the pred block. NOTE, this - // assumes that terminators do not have side effects! - // FIXME: This doesn't work for FP_REG_KILL. - - while (!Pred->empty() && TII.isTerminatorInstr(Pred->back().getOpcode())) - Pred->pop_back(); + + // If the prior block branches somewhere else on the condition and here if + // the condition is false, remove the uncond second branch. + if (PriorFBB == MBB) { + TII->RemoveBranch(PrevBB); + TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); + MadeChange = true; + ++NumBranchOpts; + return OptimizeBlock(MBB); + } + + // If the prior block branches here on true and somewhere else on false, and + // if the branch condition is reversible, reverse the branch to create a + // fall-through. + if (PriorTBB == MBB) { + std::vector NewPriorCond(PriorCond); + if (!TII->ReverseBranchCondition(NewPriorCond)) { + TII->RemoveBranch(PrevBB); + TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); + MadeChange = true; + ++NumBranchOpts; + return OptimizeBlock(MBB); + } + } + + // If this block doesn't fall through (e.g. it ends with an uncond branch or + // has no successors) and if the pred falls through into this block, and if + // it would otherwise fall through into the block after this, move this + // block to the end of the function. + // + // We consider it more likely that execution will stay in the function (e.g. + // due to loops) than it is to exit it. This asserts in loops etc, moving + // the assert condition out of the loop body. + if (!PriorCond.empty() && PriorFBB == 0 && + MachineFunction::iterator(PriorTBB) == FallThrough && + !CanFallThrough(MBB)) { + bool DoTransform = true; - // Splice the instructions over. - Pred->splice(Pred->end(), MBB, MBB->begin(), MBB->end()); + // We have to be careful that the succs of PredBB aren't both no-successor + // blocks. If neither have successors and if PredBB is the second from + // last block in the function, we'd just keep swapping the two blocks for + // last. Only do the swap if one is clearly better to fall through than + // the other. + if (FallThrough == --MBB->getParent()->end() && + !IsBetterFallthrough(PriorTBB, MBB)) + DoTransform = false; + + // We don't want to do this transformation if we have control flow like: + // br cond BB2 + // BB1: + // .. + // jmp BBX + // BB2: + // .. + // ret + // + // In this case, we could actually be moving the return block *into* a + // loop! + if (DoTransform && !MBB->succ_empty() && + (!CanFallThrough(PriorTBB) || PriorTBB->empty())) + DoTransform = false; - // If MBB does not end with a barrier, add a goto instruction to the end. - if (Pred->empty() || !TII.isBarrier(Pred->back().getOpcode())) - TII.insertGoto(*Pred, *next(MBB)); - // Update the CFG now. - Pred->removeSuccessor(Pred->succ_begin()); - while (!MBB->succ_empty()) { - Pred->addSuccessor(*(MBB->succ_end()-1)); - MBB->removeSuccessor(MBB->succ_end()-1); + if (DoTransform) { + // Reverse the branch so we will fall through on the previous true cond. + std::vector NewPriorCond(PriorCond); + if (!TII->ReverseBranchCondition(NewPriorCond)) { + DOUT << "\nMoving MBB: " << *MBB; + DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; + + TII->RemoveBranch(PrevBB); + TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); + + // Move this block to the end of the function. + MBB->moveAfter(--MBB->getParent()->end()); + MadeChange = true; + ++NumBranchOpts; + return; + } } - return true; } } - // If BB falls through into Old, insert an unconditional branch to New. - MachineFunction::iterator BBSucc = BB; ++BBSucc; - if (BBSucc != BB->getParent()->end() && &*BBSucc == Old) - TII.insertGoto(*BB, *New); - - - if (MBB->pred_size() == 1) { - // If this block has a single predecessor, and if that block has a single - // successor, merge this block into that block. - MachineBasicBlock *Pred = *MBB->pred_begin(); - if (Pred->succ_size() == 1) { - // Delete all of the terminators from end of the pred block. NOTE, this - // assumes that terminators do not have side effects! - // FIXME: This doesn't work for FP_REG_KILL. - - while (!Pred->empty() && TII.isTerminatorInstr(Pred->back().getOpcode())) - Pred->pop_back(); + // Analyze the branch in the current block. + MachineBasicBlock *CurTBB = 0, *CurFBB = 0; + std::vector CurCond; + bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); + if (!CurUnAnalyzable) { + // If the CFG for the prior block has extra edges, remove them. + MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); - // Splice the instructions over. - Pred->splice(Pred->end(), MBB, MBB->begin(), MBB->end()); + // If this is a two-way branch, and the FBB branches to this block, reverse + // the condition so the single-basic-block loop is faster. Instead of: + // Loop: xxx; jcc Out; jmp Loop + // we want: + // Loop: xxx; jncc Loop; jmp Out + if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { + std::vector NewCond(CurCond); + if (!TII->ReverseBranchCondition(NewCond)) { + TII->RemoveBranch(*MBB); + TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); + MadeChange = true; + ++NumBranchOpts; + return OptimizeBlock(MBB); + } + } + + + // If this branch is the only thing in its block, see if we can forward + // other blocks across it. + if (CurTBB && CurCond.empty() && CurFBB == 0 && + MBB->begin()->getDesc().isBranch() && CurTBB != MBB) { + // This block may contain just an unconditional branch. Because there can + // be 'non-branch terminators' in the block, try removing the branch and + // then seeing if the block is empty. + TII->RemoveBranch(*MBB); + + // If this block is just an unconditional branch to CurTBB, we can + // usually completely eliminate the block. The only case we cannot + // completely eliminate the block is when the block before this one + // falls through into MBB and we can't understand the prior block's branch + // condition. + if (MBB->empty()) { + bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); + if (PredHasNoFallThrough || !PriorUnAnalyzable || + !PrevBB.isSuccessor(MBB)) { + // If the prior block falls through into us, turn it into an + // explicit branch to us to make updates simpler. + if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && + PriorTBB != MBB && PriorFBB != MBB) { + if (PriorTBB == 0) { + assert(PriorCond.empty() && PriorFBB == 0 && + "Bad branch analysis"); + PriorTBB = MBB; + } else { + assert(PriorFBB == 0 && "Machine CFG out of date!"); + PriorFBB = MBB; + } + TII->RemoveBranch(PrevBB); + TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); + } - // If MBB does not end with a barrier, add a goto instruction to the end. - if (Pred->empty() || !TII.isBarrier(Pred->back().getOpcode())) - TII.insertGoto(*Pred, *next(MBB)); + // Iterate through all the predecessors, revectoring each in-turn. + size_t PI = 0; + bool DidChange = false; + bool HasBranchToSelf = false; + while(PI != MBB->pred_size()) { + MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); + if (PMBB == MBB) { + // If this block has an uncond branch to itself, leave it. + ++PI; + HasBranchToSelf = true; + } else { + DidChange = true; + PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); + } + } - // Update the CFG now. - Pred->removeSuccessor(Pred->succ_begin()); - while (!MBB->succ_empty()) { - Pred->addSuccessor(*(MBB->succ_end()-1)); - MBB->removeSuccessor(MBB->succ_end()-1); + // Change any jumptables to go to the new MBB. + MBB->getParent()->getJumpTableInfo()-> + ReplaceMBBInJumpTables(MBB, CurTBB); + if (DidChange) { + ++NumBranchOpts; + MadeChange = true; + if (!HasBranchToSelf) return; + } + } } - return true; + + // Add the branch back if the block is more than just an uncond branch. + TII->InsertBranch(*MBB, CurTBB, 0, CurCond); } } - // If the first instruction in this block is an unconditional branch, and if - // there are predecessors, fold the branch into the predecessors. - if (!MBB->pred_empty() && isUncondBranch(MBB->begin(), TII)) { - MachineInstr *Br = MBB->begin(); - assert(Br->getNumOperands() == 1 && Br->getOperand(0).isMachineBasicBlock() - && "Uncond branch should take one MBB argument!"); - MachineBasicBlock *Dest = Br->getOperand(0).getMachineBasicBlock(); - - while (!MBB->pred_empty()) { - MachineBasicBlock *Pred = *(MBB->pred_end()-1); - ReplaceUsesOfBlockWith(Pred, MBB, Dest, TII); - } - return true; - } + // If the prior block doesn't fall through into this block, and if this + // block doesn't fall through into some other block, see if we can find a + // place to move this block where a fall-through will happen. + if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, + PriorTBB, PriorFBB, PriorCond)) { + // Now we know that there was no fall-through into this block, check to + // see if it has a fall-through into its successor. + bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, + CurCond); - // If the last instruction is an unconditional branch and the fall through - // block is the destination, just delete the branch. - if (isUncondBranch(--MBB->end(), TII)) { - MachineBasicBlock::iterator MI = --MBB->end(); - MachineInstr *UncondBr = MI; - MachineFunction::iterator FallThrough = next(MBB); - - MachineFunction::iterator UncondDest = - MI->getOperand(0).getMachineBasicBlock(); - if (UncondDest == FallThrough) { - // Just delete the branch. This does not effect the CFG. - MBB->erase(UncondBr); - return true; + if (!MBB->isLandingPad()) { + // Check all the predecessors of this block. If one of them has no fall + // throughs, move this block right after it. + for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), + E = MBB->pred_end(); PI != E; ++PI) { + // Analyze the branch at the end of the pred. + MachineBasicBlock *PredBB = *PI; + MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; + if (PredBB != MBB && !CanFallThrough(PredBB) + && (!CurFallsThru || !CurTBB || !CurFBB) + && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { + // If the current block doesn't fall through, just move it. + // If the current block can fall through and does not end with a + // conditional branch, we need to append an unconditional jump to + // the (current) next block. To avoid a possible compile-time + // infinite loop, move blocks only backward in this case. + // Also, if there are already 2 branches here, we cannot add a third; + // this means we have the case + // Bcc next + // B elsewhere + // next: + if (CurFallsThru) { + MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); + CurCond.clear(); + TII->InsertBranch(*MBB, NextBB, 0, CurCond); + } + MBB->moveAfter(PredBB); + MadeChange = true; + return OptimizeBlock(MBB); + } + } } - - // Okay, so we don't have a fall-through. Check to see if we have an - // conditional branch that would be a fall through if we reversed it. If - // so, invert the condition and delete the uncond branch. - if (MI != MBB->begin() && isCondBranch(--MI, TII)) { - // We assume that conditional branches always have the branch dest as the - // last operand. This could be generalized in the future if needed. - unsigned LastOpnd = MI->getNumOperands()-1; - if (MachineFunction::iterator( - MI->getOperand(LastOpnd).getMachineBasicBlock()) == FallThrough) { - // Change the cond branch to go to the uncond dest, nuke the uncond, - // then reverse the condition. - MI->getOperand(LastOpnd).setMachineBasicBlock(UncondDest); - MBB->erase(UncondBr); - TII.reverseBranchCondition(MI); - return true; + + if (!CurFallsThru) { + // Check all successors to see if we can move this block before it. + for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), + E = MBB->succ_end(); SI != E; ++SI) { + // Analyze the branch at the end of the block before the succ. + MachineBasicBlock *SuccBB = *SI; + MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; + std::vector SuccPrevCond; + + // If this block doesn't already fall-through to that successor, and if + // the succ doesn't already have a block that can fall through into it, + // and if the successor isn't an EH destination, we can arrange for the + // fallthrough to happen. + if (SuccBB != MBB && !CanFallThrough(SuccPrev) && + !SuccBB->isLandingPad()) { + MBB->moveBefore(SuccBB); + MadeChange = true; + return OptimizeBlock(MBB); + } + } + + // Okay, there is no really great place to put this block. If, however, + // the block before this one would be a fall-through if this block were + // removed, move this block to the end of the function. + if (FallThrough != MBB->getParent()->end() && + PrevBB.isSuccessor(FallThrough)) { + MBB->moveAfter(--MBB->getParent()->end()); + MadeChange = true; + return; } } } -#endif }